This invention relates to the treatment of refractory metal surfaces for subsequent electroless plating of metal on the surface.
Ceramic chip carriers are rapidly gaining popularity for use in the packaging of semiconductor chips. These carriers typically include alumina-based substrates and have discrete areas of multilayer metallization bonded to the ceramic substrate. Generally, the areas of metallization comprise, in sequence, a base metallization layer bonded to the ceramic substrate, a layer of nickel bonded to the base layer, and a layer of gold bonded to the nickel layer. Tungsten and molybdenum are generally the metals of choice for the base layer. Furthermore, electroless deposition is often the method of choice for applying nickel to the base layer.
Since tungsten and molybdenum are not active metals for the electroless deposition of nickel thereon, an activation step prior to the nickel deposition is required. Prior art methods of activation generally involve the addition of an activator to the refractory material. For example, one activation method of the prior art calls for the chemisorption of palladium onto the refractory metal surface after a long series of cleaning steps. While indeed activating the surface, this process is extremely cumbersome and difficult to control.
Another method of activation employed in the past involves the mixing of an activating material such as nickel, copper or palladium to the metallized ink composition which is then applied to the ceramic sheets to form the tungsten or molybdenum layer. This method also exhibits several drawbacks. For example, severe shrinkage of the metallization may occur, causing warpage of the ceramic part and consequent delamination of the refractory metal from the ceramic surface.
It is another object of the present invention to provide a refractory surface activation method which does not require the addition of activating agents to the refractory metal.
It is still another object to provide a simplified process for altering the chemical characteristics of a tungsten or molybdenum surface layer to render it active for the subsequent electroless deposition of a metal.